Crystalline silicon ingot and method of fabricating the same

ABSTRACT

A crystalline silicon ingot and a method of fabricating the same are disclosed. The crystalline silicon ingot of the invention includes multiple silicon crystal grains growing in a vertical direction of the crystalline silicon ingot. The crystalline silicon ingot has a bottom with a silicon crystal grain having a first average crystal grain size of less than about 12 mm. The crystalline silicon ingot has an upper portion, which is about 250 mm away from said bottom, with a silicon crystal grain having a second average crystal grain size of greater than about 14 mm.

CROSS-REFERENCE TO RELATED APPLICATION

This utility application claims priority to Taiwan application serialnumber 100137420, filed Oct. 14, 2011, which is incorporated herein byreference.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The invention relates to a crystalline silicon ingot and a method offabricating the same, and more particularly, to a crystalline siliconingot containing small-sized silicon crystal grains at a bottom thereofand large-sized silicon crystal grains at a top thereof and a method offabricating the same. The background of the disclosure can refer to thefollowing references:

[1] K. Fujiwara, W. Pan, K. Sawada, M. Tokairin, N. Usami, Y. Nose, A.Nomura, T. Shishida, K. Nakajima, Directional growth method to obtainhigh quality polycrystalline silicon from its melt, Journal of CrystalGrowth 2006; 292:282-285; and

[2] T. Y. Wang, S. L. Hsu, C. C. Fei, K. M. Yei, W. C. Hsu, C. W. Lan,Grain control using spot cooling in multi-crystalline silicon crystalgrowth, Journal of Crystal Growth 2009; 311:263-267.

2. Brief Description of the Related Art

Crystal growth by casting polycrystalline silicon in a laboratory gradecan attain the growth of facet dendrite in a bottom of a crucible. Forexample, the above-mentioned reference [1] proposes crystal growth in alateral direction densely spreads on a bottom of a crucible by localundercooling, and then post-shaped structures grow upwards. Large-sizedsilicon crystal grains thereof have low defect density and a better dualcrystal structure, sigma 3. Accordingly, made from a silicon wafersliced from the crystalline silicon ingot produced in accordance withreference [1], solar cells can achieve higher photo-electron conversionefficiency.

However, in the extent of a scale for an industry grade, it isrelatively difficult to have facet dendrite densely spread on a bottomof a crucible by local undercooling. Industry-grade polycrystallinesilicon cast, affected by the crucible and the uniformity of heating theentirety, is performed with the increase of variances of controlling theinitial undercooling degree. It results in the fact that thepolycrystalline silicon grows with large-sized crystal grains andwithout any better dual crystal structure in the bottom of the crucible,so as to become a portion with higher defect density. The defect densitydramatically increases during crystal growth such that the crystallinesilicon ingot has a poor crystal quality and subsequently-formed solarcells have lower photo-electron conversion efficiency.

SUMMARY OF THE DISCLOSURE

The present invention is directed to a crystalline silicon ingot and amethod of fabricating the same. The crystalline silicon ingot is formedwith reduction of an increasing rate of defects, and thus thecrystalline silicon ingot has a better crystal quality. Also,subsequently-formed solar cells have higher photo-electron conversionefficiency.

In one embodiment for fabricating a crystalline silicon ingot, a siliconmelt can be first formed in a crucible that itself is defined with avertical direction. Next, at least one thermal control parameter of thesilicon melt is controlled such that multiple silicon crystal grains inthe silicon melt nucleate on an inner wall of a bottom of the crucibleand grow in the vertical direction. Finally, the thermal controlparameter continues to be controlled until the entirety of the siliconmelt solidifies to become a crystalline silicon ingot.

With regards to the present invention, the crystalline silicon ingot hasa bottom with a silicon crystal grain having a first average crystalgrain size of less than about 12 mm. The crystalline silicon ingot hasan upper portion, 250 mm away from the bottom thereof, with a siliconcrystal grain having a second average crystal grain size of greater thanabout 14 mm. In another embodiment, the silicon crystal grain at thebottom of the crystalline silicon ingot has the first average crystalgrain size of preferably less than about 8 mm.

With regards to the invention, the silicon crystal grain at the upperportion of the crystalline silicon ingot has a defect density less than20% in term of defect area ratio.

In one embodiment, the inner wall of the bottom of the crucible has aroughness ranging from 300 micrometers to 1000 micrometers such that theinner wall of the bottom provides multiple nucleation sites for siliconcrystal grains.

In one embodiment, a heater is mounted on the crucible, and adirectional solidification block is mounted under the crucible. Thethermal control parameter may contain a first temperature gradient fromthe heater to the crucible, a second temperature gradient from a bottomof the silicon melt to a top of the directional solidification block ora heat flux.

Different from the prior art, the present invention proposes a spreadingratio of large-sized silicon crystal grains can be dramatically reducedby controlling the thermal control parameter and the nucleation sitesdensely spreading on the bottom of the crucible. Small-sized siliconcrystal grains result in less competition of crystal growth and spreaddensely so as to be subject to growing upwards in a single direction.This reduces the phenomenon that large-sized crystal grains absorbsmall-sized ones and avoids the defect that post-shaped crystal grainscan not grow completely. Besides, a grain boundary of crystal grainsdensely spreading provides a path for movement due to in-crystaldislocation or other stress defects. This reduces a rate of increasingdefects and improves a crystal quality of the crystalline silicon ingot,as a whole. Subsequently-formed solar cells provide betterphoto-electron conversion efficiency.

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated as a part of thisspecification. The drawings illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings disclose illustrative embodiments of the presentdisclosure. They do not set forth all embodiments. Other embodiments maybe used in addition or instead. Details that may be apparent orunnecessary may be omitted to save space or for more effectiveillustration. Conversely, some embodiments may be practiced without allof the details that are disclosed. When the same numeral appears indifferent drawings, it refers to the same or like components or steps.

Aspects of the disclosure may be more fully understood from thefollowing description when read together with the accompanying drawings,which are to be regarded as illustrative in nature, and not as limiting.The drawings are not necessarily to scale, emphasis instead being placedon the principles of the disclosure.

FIGS. 1A and 1B illustrate a method for fabricating a crystallinesilicon ingot in accordance with an embodiment of the present invention.

FIG. 2 illustrates a metallography of an inner wall of a bottom of acrucible, which shows multiple protrusions formed on the inner wall ofthe bottom thereof

FIG. 3 illustrates a metallography of the inner wall of the bottom ofthe crucible before processed.

FIG. 4 illustrates a result of comparing silicon crystal grain sizes ofa crystalline silicon ingot of an embodiment of the present inventionand those of a compared crystalline silicon ingot.

FIG. 5 illustrates a result of comparing defect densities of acrystalline silicon ingot of an embodiment of the present invention andthose of a compared crystalline silicon ingot.

FIG. 6A illustrates metallographic images of silicon crystal grain sizesat a bottom, middle and top of a crystalline silicon ingot of anembodiment of the present invention.

FIG. 6B illustrates metallographic images of defect densities at abottom, middle and top of a crystalline silicon ingot of an embodimentof the present invention.

FIG. 7A illustrates metallographic images of silicon crystal grain sizesat a bottom, middle and top of a compared crystalline silicon ingot.

FIG. 7B illustrates metallographic images of defect densities at abottom, middle and top of a compared crystalline silicon ingot.

FIG. 8 illustrates a result of comparing average photo-electronconversion efficiencies of solar cells created from a crystallinesilicon ingot of an embodiment of the present invention and thosecreated from a compared crystalline silicon ingot.

While certain embodiments are depicted in the drawings, one skilled inthe art will appreciate that the embodiments depicted are illustrativeand that variations of those shown, as well as other embodimentsdescribed herein, may be envisioned and practiced within the scope ofthe present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

Illustrative embodiments are now described. Other embodiments may beused in addition or instead. Details that may be apparent or unnecessarymay be omitted to save space or for a more effective presentation.Conversely, some embodiments may be practiced without all of the detailsthat are disclosed.

The present provides a method for fabricating a crystalline siliconingot with a significantly reduced spreading ratio of large-sizedsilicon crystal grains by controlling a thermal control parameter,nucleation sites densely spread on a bottom of a crucible and so on.Besides, as a whole, the crystalline silicon ingot has a better crystalquality and subsequently-formed solar cells provide betterphoto-electron conversion efficiency.

FIGS. 1A and 1B are cross-sectional views illustrating a method forfabricating a crystalline silicon ingot in accordance with an embodimentof the present invention.

As shown in FIG. 1A, the method of the present invention is typicallybased on a directional solidification system (DSS) containing a DSScrystal growth furnace 1. The DSS crystal growth furnace 1 includes afurnace body 10, a heat insulating cage 12 composed of a top heatinsulating mask 122 and a bottom heat insulating plate 124, adirectional solidification block 18 mounted in the heat insulating cage12, at least one post 19 supporting the directional solidification block18, a pedestal 17 mounted on the directional solidification block 18, acrucible 16 mounted in the pedestal 17, a heater mounted on the crucible16 and an inert gas duct 11 passing through the furnace body 10 and theheat insulating cage 12.

In one embodiment, the crucible 16 can be a quartz crucible. Thedirectional solidification block 18 can be made from graphite. Thepedestal 17 can be made from graphite. The inert gas duct 11 cantransmit an argon gas into the heat insulating cage 12.

With regards to the invention, a silicon melt 20 is formed in thecrucible 16, as shown in FIG. 1. The crucible 16 has a verticaldirection V.

Next, at least one thermal control parameter of the silicon melt 20 iscontrolled such that multiple silicon crystal grains 22 in the siliconmelt 20 nucleate on an inner wall 162 of a bottom of the crucible 16 andgrow in the vertical direction V, as shown in FIG. 1B. The thermalcontrol parameter contains a heat flux. During crystal growth in the DSScrystal growth furnace 1, the top heat insulating mask 122 rises slowlysuch that a clearance is created to a space originally sealed by theheat insulating cage 12 and becomes a path for heat exchanging betweenan inside and outside of the heat insulating cage 12. This creates heatflux.

Finally, the thermal control parameter continues to be controlled untilthe entirety of the silicon melt 20 solidifies to become a crystallinesilicon ingot.

In one embodiment, the inner wall 162 of the bottom of the crucible 16has a roughness ranging from 300 micrometers to 1000 micrometers suchthat the inner wall 162 of the bottom provides multiple nucleation sitesfor silicon crystal grains.

In one embodiment, the method of forming the inner wall 162, of thebottom of the crucible 16, with the roughness ranging from 300micrometers to 1000 micrometers can be performed by first formingmultiple protrusions on the inner wall 162 of the bottom of the crucible16, wherein the protrusions act as the nucleation sites and cause theinner wall 162, of the bottom of the crucible 16, with the roughness.Each of the protrusions can be made from a ceramic material or a greenor sintered body of graphite.

In one embodiment, the protrusions can be a ceramic material, such asSiN, Si₃N₄, SiO₂, SiC, Al₂O₃ and/or AN, having a melting point higherthan that of silicon. The method to form the protrusions can beperformed by spray coating slurry, formed using a powder of graphite orthe above-mentioned ceramic material, on the inner wall 162, of thebottom of the crucible 16. This can form an aggregate of theabove-mentioned powder. Next, the aggregate of the powder can becalcined or sintered in a calcining or sintering temperature suitablefor formation of a powder. Thereby, a green or sintered body of theprotrusions can be formed.

In one embodiment of the above-mentioned spray coating process, thespray coating pressure may range from 40 psi to 60 psi; the slurrypressure may range from 15 psi to 30 psi; the spray coating temperaturemay range from 40 degrees Celsius to 60 degrees Celsius. FIG. 2illustrates a metallographic image of the inner wall 162 of the bottomof the crucible 16 calcined in accordance with the above-mentionedembodiment. For comparison, FIG. 3 illustrates a metallography of theinner wall 162 of the bottom of the crucible 16 before spray coated.Referring to FIG. 3, the inner wall 162 of the bottom of the crucible 16not processed by the above-mentioned spray coating process has aroughness between 50 micrometers and 100 micrometers. Referring to FIG.2, the inner wall 162 of the bottom of the crucible 16 after theabove-mentioned spray coating and calcining processes has a roughnessbetween 300 micrometers and 500 micrometers.

Alternatively, the inner wall 162 of the bottom of the crucible 16 canbe treated using a sand blasting process, and thereby the inner wall 162of the bottom of the crucible 16 has a roughness between 300 micrometersand 1000 micrometers.

Referring to FIGS. 1A and 1B, the heater 14 is mounted on the crucible16. The directional solidification block 18 is mounted under thecrucible 16 and indirectly contacts the crucible 16. The thermal controlparameter may contain a first temperature gradient from the heater 14 tothe crucible 16, a second temperature gradient from a bottom of thesilicon melt 20 to a top of the directional solidification block 18 or aheat flux. In one embodiment, the first temperature gradient isnecessarily controlled under 0.4° C./cm by enlarging a distance betweenthe heater 14 and the crucible 16 or by controlling a heatingtemperature of the heater 14 under 1410 degrees Celsius. The secondtemperature gradient is necessarily controlled under 17° C./cm byenlarging a thickness of the directional solidification block 18. Theheat flux is necessarily controlled over 3700 W/m² by accelerating theopening of the top heat insulating mask 122 up to 3 cm/hr. The purposeof controlling the second temperature gradient and the heat flux is toincrease undercooling at the bottom of the crucible 16.

Different from the prior art, the crystalline silicon ingot has a bottomwith a silicon crystal grain having a first average crystal grain sizeof less than about 12 mm. In one embodiment, the crystalline siliconingot has an upper portion, 250 mm away from the bottom thereof, with asilicon crystal grain having a second average crystal grain size ofgreater than about 14 mm. In another embodiment, the silicon crystalgrain at the bottom of the crystalline silicon ingot has the firstaverage crystal grain size of preferably less than about 8 mm.

In one embodiment, the silicon crystal grain at the upper portion of thecrystalline silicon ingot has a defect density less than 20% in term ofdefect area ratio.

FIG. 4 illustrates average crystal grain sizes with changes in heightsof the crystalline silicon ingot A of the present invention and thosewith changes in heights of the crystalline silicon ingot B, made inaccordance with reference [1], for comparison.

FIG. 5 illustrates defect densities with changes in heights of thecrystalline silicon ingot A at corner, sidewall and center portions ofthe ingot A. The defect densities in FIG. 5 are defined by ratios ofdefect areas. For comparison, FIG. 5 also illustrates ratios of defectareas with changes in heights of the crystalline silicon ingot B atcorner, sidewall and center portions of the ingot B.

FIG. 6A illustrates metallographic images of silicon crystal grain sizesat a bottom, middle and top (250 mm away from the bottom) of thecrystalline silicon ingot A. FIG. 6B illustrates metallographic imagesof defect densities at the bottom, middle and top of the crystallinesilicon ingot A. For comparison, FIG. 7A illustrates metallographicimages of silicon crystal grain sizes at a bottom, middle and top (250mm away from the bottom) of the crystalline silicon ingot B. FIG. 7Billustrates metallographic images of defect densities at the bottom,middle and top of the crystalline silicon ingot B.

FIG. 8 illustrates photo-electron conversion efficiencies of solar cellscreated from the bottom, middle and top (250 mm away from the bottom) ofthe crystalline silicon ingot A. For comparison, FIG. 8 also illustratesphoto-electron conversion efficiencies of solar cells created from thebottom, middle and top (250 mm away from the bottom) of the crystallinesilicon ingot B.

Referring to the data of FIGS. 4, 5 and 8 and the metallographic ofFIGS. 6A, 6B, 7A and 7B, the crystal of the ingot B grows withlarge-sized crystal grains and without any better dual crystal structurein the bottom of the crucible, so as to become a portion with higherdefect density. The defect density dramatically increases during crystalgrowth such that the crystalline silicon ingot has a poor crystalquality and subsequently-formed solar cells have lower photo-electronconversion efficiency. Comparing to the ingot B, the crystal growth ofthe ingot A is controlled by a thermal control parameter and nucleationsites densely spreading on the bottom of the crucible. This results insignificantly reducing a spreading ratio of large-sized silicon crystalgrains. Small-sized silicon crystal grains result in less competition ofcrystal growth and spread densely so as to be subject to growing upwardsin a single direction. This reduces the phenomenon that large-sizedcrystal grains absorb small-sized ones and avoids the defect thatpost-shaped crystal grains can not grow completely. Besides, in theingot A, a grain boundary of crystal grains densely spreading provides apath for movement due to in-crystal dislocation or other stress defects.This reduces a rate of increasing defects and improves a crystal qualityof the crystalline silicon ingot, as a whole. Subsequently-formed solarcells provide better photo-electron conversion efficiency.

Unless otherwise stated, all measurements, values, ratings, positions,magnitudes, sizes, and other specifications that are set forth in thisspecification, including in the claims that follow, are approximate, notexact. They are intended to have a reasonable range that is consistentwith the functions to which they relate and with what is customary inthe art to which they pertain. Furthermore, unless stated otherwise, thenumerical ranges provided are intended to be inclusive of the statedlower and upper values. Moreover, unless stated otherwise, all materialselections and numerical values are representative of preferredembodiments and other ranges and/or materials may be used.

The scope of protection is limited solely by the claims, and such scopeis intended and should be interpreted to be as broad as is consistentwith the ordinary meaning of the language that is used in the claimswhen interpreted in light of this specification and the prosecutionhistory that follows, and to encompass all structural and functionalequivalents thereof.

What is claimed is:
 1. A method for fabricating a crystalline siliconingot, comprising: forming a silicon melt in a crucible defining avertical direction; controlling at least one thermal control parameterof said silicon melt such that multiple silicon crystal grains in saidsilicon melt nucleate on an inner wall of a bottom of said crucible andgrow along said vertical direction; and controlling said at least onethermal control parameter continually until entirety of said siliconmelt solidifies to become said crystalline silicon ingot, wherein saidcrystalline silicon ingot has a bottom with a silicon crystal grainhaving a first average crystal grain size of less than about 12 mm. 2.The method of claim 1, wherein said crystalline silicon ingot has anupper portion, which is about 250 mm away from said bottom, with asilicon crystal grain having a second average crystal grain size ofgreater than about 14 mm.
 3. The method of claim 2, wherein said firstaverage crystal grain size is less than about 8 mm.
 4. The method ofclaim 2, wherein said silicon crystal grain at said upper portion ofsaid crystalline silicon ingot has a defect density less than 20% interm of defect area ratio.
 5. The method of claim 2, wherein said innerwall of said bottom of said crucible has a roughness ranging from 300micrometers to 1000 micrometers such that said inner wall of said bottomprovides multiple nucleation sites for silicon crystal grains.
 6. Themethod of claim 5, wherein multiple protrusions are formed on said innerwall of said bottom of said crucible, said multiple protrusions causethe roughness of the bottom of said crucible and function as saidmultiple nucleation sites.
 7. The method of claim 6, wherein each ofsaid multiple protrusions is in form of a ceramic material or a green orsintered body of a graphite.
 8. The method of claim 7, wherein saidceramic material is selected from a group consisting of SiN, Si₃N₄,SiO₂, SiC, Al₂O₃ or AN.
 9. The method of claim 5, wherein said innerwall of said bottom of said crucible is treated by a sand blastingprocess, and thereby said inner wall of said bottom of said crucible hassaid roughness.
 10. The method of claim 2, wherein a heater is mountedon said crucible, and a directional solidification block is mountedunder said crucible, said at least one thermal control parameter isselected from a group consisting of a first temperature gradient fromsaid heater to said crucible, a second temperature gradient from abottom of said silicon melt to a top of said directional solidificationblock and a heat flux.
 11. A crystalline silicon ingot having a bottomand a vertical direction, comprising multiple silicon crystal grainsgrowing along said vertical direction, wherein said bottom has a siliconcrystal grain with a first average crystal grain size of less than about12 mm.
 12. The crystalline silicon ingot of claim 11, wherein thecrystalline silicon ingot comprises an upper portion, which about 250 mmaway from said bottom, with a silicon crystal grain having a secondaverage crystal grain size of greater than about 14 mm.
 13. Thecrystalline silicon ingot of claim 12, wherein said first averagecrystal grain size is less than about 8 mm.
 14. The crystalline siliconingot of claim 12, wherein said silicon crystal grain at said upperportion of said crystalline silicon ingot has a defect density less than20% in term of defect area ratio.
 15. A crystalline silicon ingot havinga bottom and a vertical direction, comprising multiple silicon crystalgrains growing along said vertical direction, wherein said bottom has asilicon crystal grain with a first average crystal grain size of lessthan about 12 mm, wherein said crystalline silicon ingot has an upperportion, which is about 250 mm away from said bottom, with a siliconcrystal grain having a second average crystal grain size of greater thanabout 14 mm, wherein said silicon crystal grain at said upper portion ofsaid crystalline silicon ingot has a defect density less than 20% interm of defect area ratio.